This invention relates generally to relays and more specifically to solid-state relays having high current-carrying capacities.
Relays are widely used in railway traction systems as a means of remote control for a wide variety of applications. Mechanical relays have been an important component in traction systems. However, they contain moving parts which are subject to wear and tear, especially in busy systems in which the frequency of operation is high. Solid-state relays, which do not have mechanical moving parts and are therefore likely to offer improved reliability, are candidates to replace mechanical relays if they can be designed to meet the performance requirements of railway traction systems.
Solid-state relays are well-known in the electronics industry and their designs are based on various approaches. Many of them are AC types which are made up of thyristors or similar components. The AC type of solid-state relay cannot switch off by itselfxe2x80x94it must be aided by the diminishing current in an AC system.
There are a few types of DC solid-state relays on the market which are able to switch off by themselves. However, none of them can meet the requirements of a traction system. Their leakage current is very often too high, or the current rating of the switch is too low. On the other hand, all of these solid state relay switches are of the xe2x80x9cnormally openxe2x80x9d (NO) type. An NO switch remains open (non-conducting) when the solid-state relay is not energized. Also required in a railway traction system is the xe2x80x9cnormally closedxe2x80x9d (NC) type switch. The NC switch remains closed (conducting) when the solid-state relay is not energized. Generally, a single package of two NO-type switches and two NC-type switches are required for traction applications.
The invention is an improved device for opening and closing an electrical circuit and may include one or more switches of two types: normally-open switches and normally-closed switches. A normally-open switch includes at least one MOSFET assembly consisting of a plurality of MOSFETs numbered from one to N connected in parallel, the n""th MOSFET having a greater current-carrying capacity than the (nxe2x88x921)""th MOSFET, n taking on values between 2 and N. A switch control voltage in a normal voltage range causes the MOSFETs in the normally-open switch to be turned off while a switch control voltage in an activating voltage range causes the MOSFETs to be turned on. The normally-closed switch includes a pnp bipolar transistor, an npn bipolar transistor, and a means for short-circuiting the emitter-base junctions of the two transistors. The base and the collector of the npn transistor are connected respectively to the collector and the base of the pnp transistor. A switch control voltage in a normal voltage range causes the transistors to be turned on, and a switch control voltage in an activating voltage range causing the transistors to be turned off by short-circuiting the base-emitter junctions of the two transistors. The normally-open and normally-closed switches both include a means for accommodating a current flow in either direction through the switch terminals.
The normally-open switch includes a means for turning on the MOSFETs in time sequence, the n""th MOSFET being turned on after the (nxe2x88x921)""th MOSFET, n taking on values between 2 and N and N being the number of MOSFETs in each MOSFET assembly. The normally-open switch also includes a means for discharging the gates of the MOSFETs when the switch control voltage enters the normal voltage range after having been in the activating voltage range.
When the normally-open switch comprises a single MOSFET assembly, the sources and the drains of the MOSFETs are connected through a diode bridge circuit to two switch terminals for connection to an external electrical circuit. When the normally-open switch comprises two MOSFET assemblies, the sources of the MOSFETs in the two MOSFET assemblies are connected together, and the drains are connected to two switch terminals for connection to an external electrical circuit.
When the switch is a normally-closed switch, the emitters of the pnp transistor and the npn transistor are connected through a diode bridge circuit to two switch terminals for connection to an external electrical circuit. The means for short-circuiting the base-emitter junction of the npn transistor is a MOSFET while the means for short-circuiting the base-emitter junction of the pnp transistor includes a second pnp transistor bridged across the base-emitter junction of the first pnp transistor.
The solid-state relay includes two relay control terminals for receiving a relay control voltage from an external source. The relay control voltage has a normal voltage range and an activating voltage range corresponding respectively to the normal voltage range and the activating voltage range of the switch control voltage. The solid-state relay also includes a means for transforming the relay control voltage into the switch control voltage through one or more transformation stages, at least one transformation stage being coupled to the next transformation stage by a transformer.
One of the transformation stages is a DC/AC converter stage which converts an input DC voltage to the DC/AC converter stage to an AC voltage when the relay control voltage is in the activating voltage range. The DC/AC converter stage transforms the input DC voltage to an AC voltage beginning at a predetermined time period after the relay control voltage enters the activating voltage range.
The solid-state relay includes a means for suppressing the transmission of switching noise resulting from the operation of the DC/AC converter stage out of the relay""s enclosure via conductors connecting to the relay control terminals.
The power required to operate the relay is supplied by the external source through the relay control terminals. The relay further comprises a means for maintaining the current from the external source at a constant level while the relay control voltage is in the activating voltage range.
One of the transformation stages is an AC/DC converting stage which converts an AC relay control voltage or a DC relay control voltage of arbitrary polarity to a DC voltage having a predetermined polarity when the relay control voltage is in the activating voltage range. Another of the transformation stages converts the relay control voltage to a constant DC voltage when the relay control voltage is in the activating voltage range.